1. Field of the Invention
This invention relates to the field of camera systems. More particularly, this invention relates to methods and apparatus for automatically focusing cameras.
2. Art Background
A typical auto focus system for a camera achieves focus by adjusting an optical lens system to maximize high frequency components of a signal representing the image scene. Such systems maximize the high frequency components because a focused image contains more spatial high frequency components than a blurred image.
For example in a typical video camera auto focus system, a charge coupled device (CCD) image array senses an image rendered by an optical lens system. A camera signal processor receives color signals from the CCD array, and generates a luminance signal. The luminance signal is filtered by one or more band pass filters, which extract the high frequency components of the luminance signal. The filtered luminance signal from the band pass filter is coupled to a detector circuit which generates an evaluation function. The evaluation function is a measure of the high frequency components of the filtered luminance signal.
An auto focus processor maximizes the high frequency components of the luminance signal by adjusting the focus position of the optical lens system until the evaluation function is at the maximum. The auto focus processor adjusts the focus position by actuating a motor coupled to the optical lens system.
However, an auto focus method that maximizes the high frequency components of the luminance signal fails to focus on highly bright objects. Highly bright objects are objects within the image scene that saturate the CCD array. As the auto focus processor adjusts the optical lens system to maximize the evaluation function, a blurred image of the highly bright object saturates the CCD array. Saturation of the CCD array causes the band pass filters to generate a constant output as the auto focus processor adjusts the optical lens system. The constant output of the band pass filter renders the evaluation function useless for maximizing the high frequency components of the luminance signal.
Moreover, the blurred image of the highly bright object reduces in size as the auto focus processor adjusts the optical lens system toward focus. The reduction in size of the blurred image causes reduction of some types of evaluation functions. As a consequence, the auto focus method of maximizing the evaluation function actually moves the optical lens system away from the point of focus.
As will be described, the present invention is a method and apparatus for auto focusing a camera wherein the image scene may contain highly bright objects.